1. Field of the Invention
This invention relates to metallization procedures used in the manufacture of semiconductor integrated circuits.
2. Description of the Prior Art
Metallization patterns for integrated circuits are typically formed of aluminum, aluminum-silicon alloy or silicon. It is known that aluminum and silicon are soluble in each other to a certain extent. When aluminum for the metallization is deposited on the silicon semiconductor material, dissolution of the silicon into the aluminum may be sufficient to pit the silicon. In addition, aluminum alloying through pn junctions in the vicinity of the aluminum contact causes leaky junctions. Either of these effects is unacceptable in many cases. The use for the metallization of an aluminum-silicon alloy, typically having from less than one percent to about three percent of silicon therein, greatly reduces the propensity of the two materials to alloy further.
Etchants which etch aluminum primarily are not sufficient in the cases where metallization for the integrated circuits is aluminum-silicon alloy or silicon. Conductive material may remain on the wafer and form bridges between adjacent conductor lines. These bridges are due to uneven etching. The bridges may form short-circuits. It is desirable, therefore, to use an etchant which efficiently removes both aluminum and silicon in forming integrated circuit metallization patterns.
PNAF etchants for forming aluminum, aluminum-silicon alloy and silicon metallization patterns on integrated circuit wafers are disclosed in Kikuchi et al, U.S. Pat. No. 3,825,454 entitled "Method of Forming Interconnections" and in Fraser, U.S. Pat. No. 4,022,930 entitled "Multilevel Metallization for Integrated Circuits". Fluoride ions for dissolving oxides of silicon are provided in the etchants of these references by ammonium fluoride (NH.sub.4 F).
It is believed that ammonium fluoride generates hydrofluoric acid (HF) when in equilibrium with strong acids such as nitric acid. Nitric acid is used in the etchants disclosed in the referenced patents to oxidize silicon. Hydrofluoric acid is known to be a strong etchant for silicon oxide. Therefore, it is relatively difficult to control a production etching facility using the PNAF etchants disclosed in the two patents referenced above. Not only do those etchants dissolve silicon, they also readily and vigorously dissolve the silicon oxide or silicon nitride which frequently forms a required layer of dielectric material beneath the metallization pattern being formed on the wafer. It the underlying silicon oxide or silicon nitride layer is unduly thinned or etched through, electrical performance of the resulting devices is likely to be seriously degraded. Therefore, it is desirable to use an etchant and etching process for the metallization which reduces the rate at which silicon oxide and silicon nitride are dissolved. Such an etchant would also preferably increase the rate at which silicon is dissolved relative to the rate of dissolution of silicon oxide and silicon nitride.
Another difficulty exists in using the PNAF etchants disclosed in the above-referenced patents. The etch rates produced thereby do not remain sufficiently constant for a sufficiently long period of time to be used conveniently in a production mode and environment. With large numbers of parts being processed through a particular etching bath of a reasonable size, one or more of the active species in the bath will eventually become depleted. Etch rates will diminish due to this depletion. Unless etch times or temperature or both are adjusted, specified results may not be obtained. But a requirement for adjusting etch times and temperatures in a production environment introduces undesirable complexity into the operation. In the case of the prior-art PNAF etchants, it is the depletion of the available fluoride ions (F.sup.-) which tends to cause changes in the performance of the etchant. It is desirable, therefore, to use an etchant and etching process for the aluminum, aluminum-silicon alloy and silicon wherein the etchant, in reasonable quantities, tends to retain its etching properties relatively constant over a convenient period of time such as, for example, an eight-hour shift.